Implement 'keep' parameter for drop_duplicates

[deepyaman]

Close #1302

TODO:

• Add tests for 'keep' parameter
• Add doctests to demonstrate 'keep' parameter

[HyukjinKwon]

@deepyaman do you mind rebasing and syncing to the master? There are many conflicts as of 30b3334

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@49140d5). Click here to learn what that means.
The diff coverage is 91.76%.

@@            Coverage Diff            @@
##             master    #1303   +/-   ##
=========================================
  Coverage          ?   93.75%           
=========================================
  Files             ?       34           
  Lines             ?     7254           
  Branches          ?        0           
=========================================
  Hits              ?     6801           
  Misses            ?      453           
  Partials          ?        0

Impacted Files	Coverage Δ
databricks/koalas/missing/common.py	100% <ø> (ø)
databricks/koalas/frame.py	96.71% <ø> (ø)
databricks/koalas/numpy_compat.py	90.9% <100%> (ø)
databricks/koalas/missing/frame.py	100% <100%> (ø)
databricks/koalas/missing/__init__.py	100% <100%> (ø)
databricks/koalas/mlflow.py	94.87% <100%> (ø)
databricks/koalas/missing/groupby.py	100% <100%> (ø)
databricks/koalas/missing/window.py	100% <100%> (ø)
databricks/koalas/missing/indexes.py	100% <100%> (ø)
databricks/koalas/version.py	100% <100%> (ø)
... and 24 more

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[deepyaman]

I noticed another issue with Series while trying to implement this. 1cb4ba0 changed data_scols to column_scols, but that never got reflected in

koalas/databricks/koalas/frame.py

Line 2865 in 1cb4ba0

+ self._internal.data_scols)

I don't believe the column == index_column ever got activated, so it didn't matter. This PR also attempts to fix this.

[deepyaman]

@deepyaman do you mind rebasing and syncing to the master? There are many conflicts as of 30b3334

@HyukjinKwon Synced, tests added, and ready to go!

[ueshin]

Otherwise, LGTM.

[deepyaman]

Adding additional doctests decreased code coverage to fail the build. T_T

[ueshin]

nit: we prefer to use ks.from_pandas(pser) instread of ks.Series(pser).

[ueshin]

Thanks! merging.